Field of the Invention
The present invention relates to a catching device on overtravel brake devices, as described by the preamble of the independent patent claim.
Brief Description of the Related Art
In shaft hoisting in the mining industry a known problem is the overtravelling of hoisting means. The term overtravelling therein describes the situation that in shaft hoisting the hoisting cage comes to a standstill only above the pithead or the nominal loading/unloading position, i.e. the apparatus and installations in the mine building above ground. There is a risk that the cages are pulled up to the upper end of the pithead frame as a result of a failure due to their greater speed and the moving masses of the hoisting system.
In shaft hoisting technology therefore, for the case of overtravelling of a hoisting means and a possible rope breakage in the course of this, currently catch pawls are used in the area of the clear height for protecting the hoisting means from crashing, which pawls are attached independently of the hoisting means and independently of a braking device (e.g. SSA as offered by the company Siemag Tecberg). The catch pawls are as a rule mounted in the guide frame.
The catch pawls are configured such that these are opened by the hoisting means during passage of the hoisting means through the catch pawls and are subsequently close again independently, usually by force of weight. The opening is made possible/realized for example by pivotally mounting the catch pawls outside of their balance point. The closing of the catch pawls can be effected e.g. by actuation or by the own weight of the catch pawls.
In the closed state, the catch pawls rest against a firm stop, such that a hoisting means dropping down from above can be caught.
In order to illustrate the catching process, the following scenario is described:
The hoisting system overtravels and the hoisting machine is neither turned off nor braked.
The hoisting means hits an overtravel and braking device at full or partial nominal speed.
The braking device brakes the hoisting means.
The hoisting means moves up to the area of the catch pawls, with the catch pawls being opened in the process.
After the hoisting means has passed the catch pawls, these are closed or fall shut due to their own weight.
The rope tears (machine has not been turned off), e.g. after collision with the final and static bumpers at the end of the overtravel path.
After the braking process has been concluded, the hoisting means drops down in the direction of the shaft depth.
The hoisting means strikes the catch pawls.
It should be remarked that numerous further scenarios are possible.
The current catching apparatus have a number of disadvantages and problems, however.
The effect of the catching apparatus is location-dependent. This means that the catch pawls are attached outside of the system consisting of hoisting means/security-braking device (SSA). Consequently the free fall of the hoisting means must necessarily take place before the hoisting means is caught in the catch pawls. The free fall causes a collision of the hoisting means with the catch pawls, the impact being uncontrolled in the worst case. This has the disadvantageous consequence that persons or goods located in the hoisting cage are subjected to an abrupt thrusting movement, which can lead to physical injury and/or damage of the goods. Further, the catch pawls and/or the head frame of the hoisting means could be deformed to an impermissible degree upon impact, which can lead to disastrous results in turn.
Since the effect of the catch pawls is location-dependent, it can occur that, in the case of an unfavorable scenario and when the arrangement of the catch pawls is inexpedient, the rope breaks before the hoisting means has passed the catch pawls. In this case the hoisting means would crash down the depth of the shaft completely uncontrollably.
A different, dangerous scenario would be that after braking the system upon overtravelling, the catch pawls are not reached and the braking system of the hoisting machine is ineffective; also in this case, the hoisting means would move down the shaft in uncontrolled fashion due to the gravitational forces.
When the catch pawls are arranged far down in the area of the clear height, e.g. to ensure a sufficient safety distance from the height where the hoisting means comes to a standstill, to the height where already closed catch pawls rest, there is a risk in the case of the scenario where the free fall is disproportionately high that the impact will consequently cause substantial damage to persons and/or goods.
Moreover, for the known solutions special fastening structures are required for the catch pawls, thereby causing additional costs.